# ASTRAL-Next System Component Decomposition Plan

## Design Principle: Interface-Based Architecture

**CRITICAL REQUIREMENT**: Each component MUST implement a well-defined interface to ensure interchangeability with different implementations.

**Benefits**:

- Swap implementations (e.g., replace LiteSAM with TransFG, GTSAM with Ceres)
- Enable unit testing with mocks
- Support multiple backends (TensorRT vs ONNX, different databases)
- Facilitate future enhancements without breaking contracts

**Interface Specification**: Each component spec must define:

- Interface name (e.g., `ISatelliteDataManager`, `IMetricRefinement`)
- All public methods with strict contracts  
- Input/output data structures
- Error conditions and exceptions
- Performance guarantees

---

## System Architecture Overview

**Single unified Flight API:**

- Flight CRUD operations (create, read, update, delete)
- Waypoint management within flights
- Geofence management
- Tri-layer localization (SuperPoint+LightGlue, DINOv2, LiteSAM)
- Calls satellite provider for tiles
- Rotation preprocessing (LiteSAM 45° limit)
- Per-frame waypoint updates
- Progressive tile search (1→4→9→16→25)
- SSE streaming for real-time results

---

## FLIGHT API COMPONENTS (17 components)

### Core API Layer

**F01_flight_api**
**Interface**: `IFlightAPI`
**Endpoints**: `POST /flights`, `GET /flights/{flightId}`, `DELETE /flights/{flightId}`, `PUT /flights/{flightId}/waypoints/{waypointId}`, `POST .../images/batch`, `POST .../user-fix`, `GET .../status`, `GET .../stream`

**F02.1_flight_lifecycle_manager**
**Interface**: `IFlightLifecycleManager`
**API**: `create_flight()`, `get_flight()`, `get_flight_state()`, `delete_flight()`, `update_waypoint()`, `batch_update_waypoints()`, `validate_waypoint()`, `validate_geofence()`, `queue_images()`, `handle_user_fix()`, `create_client_stream()`, `convert_object_to_gps()`, `initialize_system()`

**F02.2_flight_processing_engine**
**Interface**: `IFlightProcessingEngine`
**API**: `start_processing()`, `stop_processing()`, `process_frame()`, `apply_user_fix()`, `handle_tracking_loss()`, `get_active_chunk()`, `create_new_chunk()`

**F03_flight_database**
**Interface**: `IFlightDatabase`
**API**: `insert_flight()`, `update_flight()`, `query_flights()`, `get_flight_by_id()`, `delete_flight()`, `get_waypoints()`, `insert_waypoint()`, `update_waypoint()`, `batch_update_waypoints()`, `save_flight_state()`, `load_flight_state()`, `save_frame_result()`, `get_frame_results()`, `save_heading()`, `get_heading_history()`, `get_latest_heading()`, `save_image_metadata()`, `get_image_path()`, `get_image_metadata()`

### Data Management

**F04_satellite_data_manager**
**Interface**: `ISatelliteDataManager`
**API**: `fetch_tile()`, `fetch_tile_grid()`, `prefetch_route_corridor()`, `progressive_fetch()`, `cache_tile()`, `get_cached_tile()`, `compute_tile_coords()`, `expand_search_grid()`, `compute_tile_bounds()`
**Features**: Progressive retrieval, tile caching, grid calculations

**F05_image_input_pipeline**
**Interface**: `IImageInputPipeline`
**API**: `queue_batch()`, `process_next_batch()`, `validate_batch()`, `store_images()`, `get_next_image()`, `get_image_by_sequence()`
**Features**: FIFO queuing, validation, storage

**F06_image_rotation_manager**
**Interface**: `IImageRotationManager`
**API**: `rotate_image_360()`, `try_rotation_steps()`, `calculate_precise_angle()`, `get_current_heading()`, `update_heading()`, `detect_sharp_turn()`, `requires_rotation_sweep()`
**Features**: 30° rotation sweeps, heading tracking

### Visual Processing

**F07_sequential_visual_odometry**
**Interface**: `ISequentialVisualOdometry`
**API**: `compute_relative_pose()`, `extract_features()`, `match_features()`, `estimate_motion()`

**F08_global_place_recognition**
**Interface**: `IGlobalPlaceRecognition`
**API**: `retrieve_candidate_tiles()`, `compute_location_descriptor()`, `query_database()`, `rank_candidates()`

**F09_metric_refinement**
**Interface**: `IMetricRefinement`
**API**: `align_to_satellite(uav_image, satellite_tile, tile_bounds)`, `compute_homography()`, `extract_gps_from_alignment()`, `compute_match_confidence()`

### State Estimation

**F10_factor_graph_optimizer**
**Interface**: `IFactorGraphOptimizer`
**API**: `add_relative_factor()`, `add_absolute_factor()`, `add_altitude_prior()`, `optimize()`, `get_trajectory()`, `get_marginal_covariance()`

**F11_failure_recovery_coordinator**
**Interface**: `IFailureRecoveryCoordinator`
**API**: `check_confidence()`, `detect_tracking_loss()`, `start_search()`, `expand_search_radius()`, `try_current_grid()`, `create_user_input_request()`, `apply_user_anchor()`

**F12_route_chunk_manager**
**Interface**: `IRouteChunkManager`
**API**: `create_chunk()`, `add_frame_to_chunk()`, `get_chunk_frames()`, `get_chunk_images()`, `get_chunk_composite_descriptor()`, `get_chunk_bounds()`, `is_chunk_ready_for_matching()`, `mark_chunk_anchored()`, `get_chunks_for_matching()`, `get_active_chunk()`, `deactivate_chunk()`
**Features**: Chunk lifecycle management, chunk state tracking, chunk matching coordination

**F13_coordinate_transformer**
**Interface**: `ICoordinateTransformer`
**API**: `set_enu_origin()`, `get_enu_origin()`, `gps_to_enu()`, `enu_to_gps()`, `pixel_to_gps()`, `gps_to_pixel()`, `image_object_to_gps()`, `transform_points()`
**Dependencies**: F10 Factor Graph Optimizer (for frame poses), H02 GSD Calculator (for GSD computation)

### Results & Communication

**F14_result_manager**
**Interface**: `IResultManager`
**API**: `update_frame_result()`, `publish_waypoint_update()`, `get_flight_results()`, `mark_refined()`

**F15_sse_event_streamer**
**Interface**: `ISSEEventStreamer`
**API**: `create_stream()`, `send_frame_result()`, `send_search_progress()`, `send_user_input_request()`, `send_refinement()`

### Infrastructure

**F16_model_manager**
**Interface**: `IModelManager`
**API**: `load_model()`, `get_inference_engine()`, `optimize_to_tensorrt()`, `fallback_to_onnx()`

**F17_configuration_manager**
**Interface**: `IConfigurationManager`
**API**: `load_config()`, `get_camera_params()`, `validate_config()`, `get_flight_config()`

---

## HELPER COMPONENTS (8 components)

**H01_camera_model** - `ICameraModel`
**H02_gsd_calculator** - `IGSDCalculator`
**H03_robust_kernels** - `IRobustKernels`
**H04_faiss_index_manager** - `IFaissIndexManager`
**H05_performance_monitor** - `IPerformanceMonitor`
**H06_web_mercator_utils** - `IWebMercatorUtils`
**H07_image_rotation_utils** - `IImageRotationUtils`
**H08_batch_validator** - `IBatchValidator`

---

## System Startup Initialization Order

**Startup sequence** (blocking, sequential):

| Order | Component | Method | Purpose | Dependencies |
|-------|-----------|--------|---------|--------------|
| 1 | F17 Configuration Manager | `load_config()` | Load system configuration | None |
| 2 | F03 Flight Database | Initialize connections | Establish DB connection pool | F17 |
| 3 | F16 Model Manager | `load_model("SuperPoint")` | Load SuperPoint feature extractor | F17 |
| 4 | F16 Model Manager | `load_model("LightGlue")` | Load LightGlue matcher | F17 |
| 5 | F16 Model Manager | `load_model("DINOv2")` | Load DINOv2 for place recognition | F17 |
| 6 | F16 Model Manager | `load_model("LiteSAM")` | Load LiteSAM for cross-view matching | F17 |
| 7 | F04 Satellite Data Manager | Initialize cache | Initialize tile cache directory | F17 |
| 8 | F08 Global Place Recognition | `load_index()` | Load pre-built Faiss index from satellite provider | F04, F16, H04 |
| 9 | F12 Route Chunk Manager | Initialize | Initialize chunk state tracking | F10 |
| 10 | F02 Flight Processor | Ready | Ready to accept flights | All above |
| 11 | F01 Flight API | Start server | Start FastAPI/Uvicorn | F02 |

**Estimated total startup time**: ~30 seconds (dominated by model loading)

**Shutdown sequence** (reverse order):
1. F01 Flight API - Stop accepting requests
2. F02 Flight Processor - Complete or cancel active flights
3. F11 Failure Recovery Coordinator - Stop background chunk matching
4. F12 Route Chunk Manager - Save chunk state
5. F16 Model Manager - Unload models
6. F03 Flight Database - Close connections
7. F04 Satellite Data Manager - Flush cache

---

## Comprehensive Component Interaction Matrix

### System Initialization

| Source | Target | Method | Purpose |
|--------|--------|--------|---------|
| F02.1 | F17 | `load_config()` | Load system configuration |
| F02.1 | F16 | `load_model()` × 4 | Load SuperPoint, LightGlue, DINOv2, LiteSAM |
| F04 | F08 | Satellite tiles + index | F08 loads pre-built Faiss index from provider |
| F08 | H04 | `load_index()` | Load satellite descriptor index |
| F08 | F16 | `get_inference_engine("DINOv2")` | Get model for descriptor computation |

### Flight Creation

| Source | Target | Method | Purpose |
|--------|--------|--------|---------|
| Client | F01 | `POST /flights` | Create flight |
| F01 | F02.1 | `create_flight()` | Initialize flight state |
| F02.1 | F17 | `get_flight_config()` | Get camera params, altitude |
| F02.1 | F13 | `set_enu_origin(flight_id, start_gps)` | Set ENU coordinate origin |
| F02.1 | F04 | `prefetch_route_corridor()` | Prefetch tiles |
| F04 | Satellite Provider | `GET /api/satellite/tiles/batch` | HTTP batch download (includes tile metadata) |
| F04 | H06 | `compute_tile_bounds()` | Tile coordinate calculations |
| F02.1 | F03 | `insert_flight()` | Persist flight data |

### SSE Stream Creation

| Source | Target | Method | Purpose |
|--------|--------|--------|---------|
| Client | F01 | `GET .../stream` | Open SSE connection |
| F01 | F02.1 | `create_client_stream()` | Route through lifecycle manager |
| F02.1 | F15 | `create_stream()` | Establish SSE channel |

### Image Upload

| Source | Target | Method | Purpose |
|--------|--------|--------|---------|
| Client | F01 | `POST .../images/batch` | Upload 10-50 images |
| F01 | F02.1 | `queue_images()` | Route through lifecycle manager |
| F02.1 | F05 | `queue_batch()` | Queue for processing |
| F05 | H08 | `validate_batch()` | Validate sequence, format |
| F05 | F03 | `save_image_metadata()` | Persist image metadata |

### Per-Frame Processing (First Frame / Sharp Turn)

| Source | Target | Method | Purpose |
|--------|--------|--------|---------|
| F02.2 | F05 | `get_next_image()` | Get image for processing |
| F02.2 | F06 | `requires_rotation_sweep()` | Check if sweep needed |
| F06 | H07 | `rotate_image()` × 12 | Rotate in 30° steps |
| F06 | F09 | `align_to_satellite(img, tile, bounds)` × 12 | Try LiteSAM each rotation |
| F02.2 | F04 | `get_cached_tile()` + `compute_tile_bounds()` | Get expected tile with bounds |
| F09 | F16 | `get_inference_engine("LiteSAM")` | Get model |
| F06 | H07 | `calculate_rotation_from_points()` | Precise angle from homography |
| F02.2 | F03 | `save_heading()` | Store UAV heading |

### Per-Frame Processing (Sequential VO)

| Source | Target | Method | Purpose |
|--------|--------|--------|---------|
| F02.2 | F12 | `get_active_chunk()` | Get active chunk for frame |
| F02.2 | F07 | `compute_relative_pose()` | Provide image and chunk context |
| F07 | F16 | `get_inference_engine("SuperPoint")` | Get feature extractor |
| F07 | F16 | `get_inference_engine("LightGlue")` | Get matcher |
| F07 | H05 | `start_timer()`, `end_timer()` | Monitor timing |
| F02.2 | F10 | `add_relative_factor_to_chunk()` | Add pose measurement to chunk subgraph |
| F02.2 | F12 | `add_frame_to_chunk()` | Add frame to chunk |

### Tracking Good (Drift Correction)

| Source | Target | Method | Purpose |
|--------|--------|--------|---------|
| F02.2 | F11 | `check_confidence()` | Check tracking quality |
| F02.2 | F04 | `fetch_tile()` + `compute_tile_bounds()` | Get single tile with bounds |
| F02.2 | F09 | `align_to_satellite(img, tile, bounds)` | Align to 1 tile |
| F02.2 | F10 | `add_absolute_factor()` | Add GPS measurement |

### Tracking Lost (Progressive Search + Chunk Building)

| Source | Target | Method | Purpose |
|--------|--------|--------|---------|
| F02.2 | F11 | `check_confidence()` → FAIL | Low confidence |
| F11 | F12 | `create_chunk_on_tracking_loss()` | **Proactive chunk creation** |
| F12 | F10 | `create_chunk_subgraph()` | Create chunk in factor graph |
| F12 | F03 | `save_chunk_state()` | Persist chunk state for recovery |
| F02.2 | F12 | `get_active_chunk()` | Get new active chunk |
| F11 | F06 | `requires_rotation_sweep()` | Trigger rotation sweep (single-image) |
| F11 | F08 | `retrieve_candidate_tiles()` | Coarse localization (single-image) |
| F08 | F16 | `get_inference_engine("DINOv2")` | Get model |
| F08 | H04 | `search()` | Query Faiss index |
| F08 | F04 | `get_tile_by_gps()` × 5 | Get candidate tiles |
| F11 | F04 | `expand_search_grid(4)` | Get 2×2 grid |
| F11 | F09 | `align_to_satellite(img, tile, bounds)` | Try LiteSAM on tiles |
| F11 (fail) | F04 | `expand_search_grid(9)` | Expand to 3×3 |
| F11 (fail) | F04 | `expand_search_grid(16)` | Expand to 4×4 |
| F11 (fail) | F04 | `expand_search_grid(25)` | Expand to 5×5 |
| F11 (fail) | F12 | Continue building chunk | **Chunk building continues** |
| F11 (background) | F12 | `get_chunks_for_matching()` | Get unanchored chunks |
| F11 (background) | F08 | `retrieve_candidate_tiles_for_chunk()` | **Chunk semantic matching** |
| F11 (background) | F06 | `try_chunk_rotation_steps()` | **Chunk rotation sweeps** |
| F11 (background) | F09 | `align_chunk_to_satellite()` | **Chunk LiteSAM matching** |
| F11 (background) | F12 | `mark_chunk_anchored()` + `merge_chunks(main, new)` | **Chunk merging via F12** |
| F11 (fail) | F02.2 | Returns `UserInputRequest` | Request human help (last resort) |
| F02.2 | F15 | `send_user_input_request()` | Send SSE event to client |

### Optimization & Results

| Source | Target | Method | Purpose |
|--------|--------|--------|---------|
| F10 | H03 | `huber_loss()`, `cauchy_loss()` | Apply robust kernels |
| F10 | Internal | `optimize()` | Run iSAM2 optimization |
| F02.2 | F10 | `get_trajectory()` | Get optimized poses |
| F02.2 | F13 | `enu_to_gps()` | Convert ENU to GPS |
| F13 | H01 | `project()`, `unproject()` | Camera operations |
| F13 | H02 | `compute_gsd()` | GSD calculations |
| F13 | H06 | `tile_to_latlon()` | Coordinate transforms |
| F02.2 | F14 | Frame GPS + object coords | Provide results |
| F14 | F03 | `update_waypoint()` | Per-frame waypoint update |
| F14 | F15 | `send_frame_result()` | Publish to client |
| F15 | Client | SSE `frame_processed` | Real-time delivery |
| F14 | F03 | `save_frame_result()` | Persist frame result |

### User Input Recovery

| Source | Target | Method | Purpose |
|--------|--------|--------|---------|
| F15 | Client | SSE `user_input_needed` | Notify client |
| Client | F01 | `POST .../user-fix` | Provide anchor |
| F01 | F02.1 | `handle_user_fix()` | Route through lifecycle manager |
| F02.1 | F02.2 | `apply_user_fix()` | Delegate to processing engine |
| F02.2 | F11 | `apply_user_anchor()` | Apply fix |
| F11 | F10 | `add_absolute_factor()` (high confidence) | Hard constraint |
| F10 | Internal | `optimize()` | Re-optimize |
| F02.2 | F15 | `send_frame_result()` | Publish result via SSE |

### Asynchronous Refinement

| Source | Target | Method | Purpose |
|--------|--------|--------|---------|
| F10 | Internal (background) | `optimize()` | Back-propagate anchors |
| F02.2 | F10 | `get_trajectory()` | Get refined poses (ENU) |
| F02.2 | F13 | `enu_to_gps()` | Convert ENU poses to GPS |
| F02.2 | F14 | `mark_refined(List[RefinedFrameResult])` | Pass GPS-converted results |
| F14 | F03 | `batch_update_waypoints()` | Batch update waypoints |
| F14 | F15 | `send_refinement()` × N | Send updates |
| F15 | Client | SSE `frame_refined` × N | Incremental updates |

**Note**: F14 does NOT call F10 or F13. F02.2 performs coordinate conversion and provides GPS results to F14.

### Chunk Matching (Background)

| Source | Target | Method | Purpose |
|--------|--------|--------|---------|
| F11 (background) | F12 | `get_chunks_for_matching()` | Get unanchored chunks ready for matching |
| F11 | F12 | `get_chunk_images()` | Get chunk images |
| F12 | F05 | `get_image_by_sequence()` | **Load images for chunk** (F12 delegates to F05 for actual image retrieval) |
| F11 | F08 | `retrieve_candidate_tiles_for_chunk()` | Chunk semantic matching (aggregate DINOv2) |
| F08 | F16 | `get_inference_engine("DINOv2")` | Get model for descriptor computation |
| F08 | H04 | `search()` | Query Faiss with chunk descriptor |
| F11 | F06 | `try_chunk_rotation_steps()` | Chunk rotation sweeps (12 rotations) |
| F06 | F09 | `align_chunk_to_satellite()` × 12 | Try LiteSAM for each rotation |
| F11 | F10 | `add_chunk_anchor()` | Anchor chunk with GPS |
| F11 | F12 | `merge_chunks(main_chunk, new_chunk)` | Merge new_chunk into main_chunk (Sim3 transform) |
| F10 | Internal | `optimize_global()` | Global optimization after merging |
| F11 | F12 | `mark_chunk_anchored()` | Update chunk state |

### Cross-Cutting Concerns

| Source | Target | Method | Purpose |
|--------|--------|--------|---------|
| F17 | ALL | `get_*_config()` | Provide configuration |
| H05 | F07, F08, F09, F10, F11 | `start_timer()`, `end_timer()` | Performance monitoring |

---

## Interaction Coverage Verification

✅ **Initialization**: F02.1→F16, F17; F04→F08→H04
✅ **Flight creation**: Client→F01→F02.1→F04,F12,F16,F17,F14
✅ **Image upload**: Client→F01→F02.1→F05→H08
✅ **Rotation sweep**: F06→H07,F09 (12 iterations)
✅ **Sequential VO**: F02.2→F07→F16,F10(chunk),F12,H05
✅ **Drift correction**: F02.2→F04,F09,F10
✅ **Tracking loss**: F02.2→F11→F12(proactive chunk),F06,F08,F04(progressive),F09
✅ **Chunk building**: F02.2→F12→F10,F07
✅ **Chunk image retrieval**: F12→F05(get_image_by_sequence for chunk images)
✅ **Chunk semantic matching**: F11→F12→F08(chunk descriptor)
✅ **Chunk LiteSAM matching**: F11→F06(chunk rotation)→F09(chunk alignment)
✅ **Chunk merging**: F11→F10(Sim3 transform)
✅ **Global PR**: F08→F16,H04,F04
✅ **Optimization**: F10→H03(chunk optimization, global optimization)
✅ **Coordinate transform**: F13→F10,H01,H02,H06
✅ **Results**: F02.2→F13→F14→F15,F03
✅ **User input**: Client→F01→F02.1→F02.2→F11→F10
✅ **Refinement**: F10→F14→F03,F15
✅ **Configuration**: F17→ALL
✅ **Performance**: H05→processing components

**All major component interactions are covered.**

---

## Deliverables

**Component Count**: 25 total

- Flight API: 17 (F01-F17)
- Helpers: 8 (H01-H08)

**For each component**, create `docs/02_components/[##]_[component_name]/[component_name]_spec.md`:

1. **Interface Definition** (interface name, methods, contracts)
2. **Component Description** (responsibilities, scope)
3. **API Methods** (inputs, outputs, errors, which components call it, test cases)
4. **Integration Tests**
5. **Non-Functional Requirements** (performance, accuracy targets)
6. **Dependencies** (which components it calls)
7. **Data Models**

### Component Specifications Status

- [x] F01 Flight API (merged from R01 Route REST API)
- [x] F02.1 Flight Lifecycle Manager (split from F02 Flight Processor)
- [x] F02.2 Flight Processing Engine (split from F02 Flight Processor)
- [x] F03 Flight Database (merged from R04, G17)
- [x] F04 Satellite Data Manager
- [x] F05 Image Input Pipeline
- [x] F06 Image Rotation Manager
- [x] F07 Sequential Visual Odometry
- [x] F08 Global Place Recognition
- [x] F09 Metric Refinement
- [x] F10 Factor Graph Optimizer
- [x] F11 Failure Recovery Coordinator
- [x] F12 Route Chunk Manager (Atlas multi-map chunk lifecycle)
- [x] F13 Coordinate Transformer (with ENU origin)
- [x] F14 Result Manager
- [x] F15 SSE Event Streamer
- [x] F16 Model Manager
- [x] F17 Configuration Manager
- [x] Helper components (H01-H08)

---

## Architecture Notes

### F02 Split: Lifecycle Manager + Processing Engine

F02 has been split into two components with clear Single Responsibility:

**F02.1 Flight Lifecycle Manager**:
- Flight CRUD operations
- System initialization
- API request routing
- SSE stream creation
- User fix delegation

**F02.2 Flight Processing Engine**:
- Main processing loop
- Frame-by-frame processing orchestration  
- Recovery coordination with F11
- Chunk management coordination with F12
- Runs in background thread per active flight

**Rationale**: This split aligns with Single Responsibility Principle. F02.1 handles external-facing operations (API layer), while F02.2 handles internal processing (background engine).

**Decision**: Keep as single component. Clear internal organization with separate methods for state vs processing concerns. Event-based communication with F11 keeps recovery logic separate.

### Error Recovery Strategy

**Per-Component Recovery**:
- **F02**: Persists flight state via F03 on each significant update. On restart, loads last known state.
- **F07**: Stateless - reprocesses frame if VO fails
- **F10**: Factor graph state persisted periodically. On restart, rebuilds from F03 checkpoint.
- **F11**: Chunk state persisted via F12→F03. Recovery continues from last chunk state.
- **F12**: All chunk state persisted to F03. Restores chunk handles on restart.

**System-Wide Recovery**:
1. On crash, F02 loads flight state from F03
2. F12 restores chunk state from F03  
3. Processing resumes from last successfully processed frame
4. Incomplete chunks continue building/matching

**Event Recovery**:
- Events are fire-and-forget (no persistence)
- Subscribers rebuild state from F03 on restart

### Error Propagation Strategy

**Principle**: Errors propagate upward through the component hierarchy. Lower-level components throw exceptions or return error results; higher-level coordinators handle recovery.

**Error Propagation Chain**:
```
H01-H08 (Helpers) → F07/F08/F09 (Visual Processing) → F11 (Recovery) → F02 (Coordinator) → F01 (API) → Client
                                                                  ↓
                                                               Events → F14 → F15 → SSE → Client
```

**Error Categories**:
1. **Recoverable** (F11 handles):
   - Tracking loss → Progressive search
   - Low confidence → Rotation sweep
   - Chunk matching fails → User input request

2. **Propagated to Client** (via SSE):
   - User input needed → `user_input_needed` event
   - Processing blocked → `processing_blocked` event
   - Flight completed → `flight_completed` event

3. **Fatal** (F02 handles, returns to F01):
   - Database connection lost → HTTP 503
   - Model loading failed → HTTP 500
   - Invalid configuration → HTTP 400

**User Fix Flow**:
```
Client ---> F01 (POST /user-fix) ---> F02.handle_user_fix() ---> F11.apply_user_anchor()
                                                                      ↓
                                                              F10.add_chunk_anchor()
                                                                      ↓
                                                              F02 receives UserFixApplied event
                                                                      ↓
                                                              F14.publish_result() ---> F15 ---> SSE ---> Client
```
